Alternative Separation Sequences

Type First split Second split Third split 

In the case of a four-component mixture ABCD there are five possible sequences, each of three columns, as shown in Table 3.12. In the direct sequence all the components, except the heaviest, are taken as top products. In the indirect sequence all the components are obtained as bottoms, except the lightest In equal split both A and C are obtained as overhead, while B and D as bottoms. There also two mixed sequences, such as direct/indirect and indirect/direct , the second split making the difference. 

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Figure 1.4. Alternative Separation Sequences for Quaternary Mixture Using Continuous Distillation Columns...

Fig. 4. Two alternative separation sequences for. separating a mixture of four species.

Also, if there are two separators, the order of separation can change. The tradeoffs for these two alternative flowsheets will be different. The choice between different separation sequences can be made using the methods described in Chap. 5. However, we should be on guard to the fact that as the reactor conversion changes, the most appropriate sequence also can change. In other words, different separation system structures become appropriate for different reactor conversions. 

The significant contribution of the utilities to the total cost of a distillation-based separation sequence provides an incentive to synthesize the sequences of distillation columns which feature heat integration alternatives. 

Floquet et al. (1985) proposed a tree searching algorithm in order to synthesize chemical processes involving reactor/separator/recycle systems interlinked with recycle streams. The reactor network of this approach is restricted to a single isothermal CSTR or PFR unit, and the separation units are considered to be simple distillation columns. The conversion of reactants into products, the temperature of the reactor, as well as the reflux ratio of the distillation columns were treated as parameters. Once the values of the parameters have been specified, the composition of the outlet stream of the reactor can be estimated and application of the tree searching algorithm on the alternative separation tasks provides the less costly distillation sequence. The problem is solved for several values of the parameters and conclusions are drawn for different regions of operation. 

In the two-step process the two reactors are coupled by the same separation system. Phenol gives azeotropes with both cyclohexanone and cyclohexanol. The relative volatility of cyclohexanone to cyclohexanol is very low at normal pressure, but it rises significantly under high vacuum. Alternative separation schemes are evaluated based on direct and indirect sequences. Both are equivalent in energy consumption, although the indirect sequence is more suitable by a decoupling effect. 

The above separation sequence, although the most used in industry, is not unique. Another possibility would consist of adopting the indirect sequence . In this case, EDC separates in the first split as bottoms, followed by VCM/HC1 distillation, as pictured in Figure 7.7 (left-hand). This alternative is penalized by excessive bottom temperature in the first split at pressures above 5 bar. In addition, in the second step an intermediate compression would be necessary for an efficient separation HC1/VCM. 

Heuristic methods were developed by well-experienced engineers and researchers. The first attempt to develop a systematic heuristic approach for the synthesis of multicomponent separation sequences was made by Siirola and Rudd. Common example is hierarchical heuristic approach.Heuristic rules are applied at five design levels to generate and evaluate the alternatives using economic criteria. The hierarchical heuristic method emphasizes the strategy of decomposition and screening. It allows for a quick location of flowsheet structures that are often near ... 

While process sym basis research has not yet produced the so In lion to the problem of generation of die best flowsheet, a number of heuristic rules have been stated by various process synthesis researchers which can be of considerable aid in guiding the selection of the near optimum or base case and Tor systematically choosing those alternate sequences to be investigated. Some of the heuristic roles developed will be compering and in some cases even contradictory, but nevertheless they do serve to reduce the enormous amount of work (hat would be involvnd in evaluating all possible separation sequences. 

Example 14.2. Consider the separation problem studied by Hendry and Hughes as shown in Fig. 1.16. Determine the feasibility of two-product, ordinary distillation (method I), select an alternative separation technique (method II), if necessary, and forbid impractical splits. Determine the feasible separation subproblems and the number of possible separation sequences that incorporate only these subproblems. 

Dahl studied the 1-1 C6F6 l,3,5-(CH3)3CeH3 compound at 233 K, well below the transition temperature and observed an interplane separation of 356 pm, a value close to the sum of the van der Waals radii. The hexamethylbenzene compound was found to form two completely different crystal structures above and below its transition temperature of 273 K. This compound in the high-temperature form is trigonal in structure with an interplane separation of 356 pm. Dahl observed that this phase showed evidence of some disorder in the regular alternate stacking sequence. Below 273 K the compound appeared to be completely ordered, triclinic in crystal habit, and to possess a plane-to-plane separation of only 343 pm. 

An objective is to examine the two separation sequences shown. In the direct sequence, valves A and D are open, B and C are closed, and product 1 (H2 and CH4) is recovered in the distillate of the first tower. Alternatively, in the indirect sequence, valves B and C arc open, and product 3 (C7H8 and C12H10) is recovered in the bottoms product of the first tower. 

Several alternative distillation sequences are being examined for the separation of a mixture of light hydrocarbons. The sequences are to be compared on the basis of annualized cost, discussed below in... 

Fig. 32 (a) Perfect phase separation of side chains in a binary (A, B) copolymer bottle bmsh with alternating grafting sequence ABAB... of side chains along the backbone into a Janus cylinder structure implies formation of an AB-interface phase (shaded) between the A-rich part (bottom) and the B-rich part (top) of the cylindrical brush. The local orientation of the interface can be characterized by a unit vector oriented normal to it (arrows), (b) At nonzero but low temperatures, phase separation will occur locally, but entropy will lead to long wavelength fluctuations of the... 

After identification of several preferable sequences, choosing among the optimum sequences, taking into consideration possible thermodinamic improvements and thermal integration of columns, arises. This task is similar to the synthesis of separation flowsheets of zeotropic mixtures (see Section 8.3), and it should be solved by the same methods (i.e., by means of comparative estimation of expenditures on separation). The methods of design calculation, described in Chapters 5 7 for the modes of minimum reflux and reflux bigger than minimum, have to be used for this purpose. In contrast to zeotropic mixtures, the set of alternative preferable sequences for azeotropic mixtures that sharply decreases the volume of necessary calculation is much smaller. 

Having derived the networks for the separation problem, a mathematical programming formulation is presented for each network to select the optimum flowsheet alternative of the separation sequence. 

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